The capacity of public policies to address wicked problems depends on them fostering innovation in both ideational terms and in practice. The public policy literature highlights the importance of policy subsystems in shaping the conditions for policy innovation. However, while collaborative and networked governance has been shown to facilitate innovation, less attention has been paid to how leadership within policy subsystems and their composition influence ideational change, especially in weak or marginal policy domains. This paper addresses that gap by examining youth policy in Catalonia: a paradigmatic case of a wicked problem that is not a high priority for the administration. Despite a history of practical innovation (e.g. cross-sectoral collaboration and participatory methods), youth policy has struggled to achieve ideational renewal. By comparing four local youth policies in two Catalan cities, we explore how subsystem dynamics (collaboration in terms of breadth, scope, depth, and leadership) affect the capacity for innovation at both practical and ideational level, and ask to what extent their composition and leadership shape innovation in a marginal policy field. Findings show that broad, well-managed collaboration, especially under strong central leadership, enhances policy innovation and helps address the marginality of youth policy.